Molecular basis for the species selectivity of the neurokinin-1 receptor antagonists CP-96,345 and RP67580

Molecular and pharmacological characterization of the murine tachykinin NK(3) receptor

Starting with a partial sequence from Genbank, polymerase chain reaction (PCR) was utilized to isolate the full-length cDNA for NK(3) receptor from mouse brain. The murine NK(3) receptor has a predicted sequence of 452 amino acids, sharing 96% and 86% identity to the rat and human NK(3) receptors, respectively. Binding affinities and functional potencies of tachykinin receptor agonists were similar in HEK (human embryonic kidney) 293 cells expressing murine NK(3) receptor and human NK(3) receptor, although substance P and neurokinin A were more potent stimulators of Ca(2+) mobilization in murine NK(3) receptor cells. NK(3) receptor-selective antagonists from two structural classes, had 10- to 100-fold lower binding affinities for murine NK(3) receptor compared to human NK(3) receptor, and about 5- to 10-fold reduced potency in the murine NK(3) receptor functional assay. The results demonstrate species differences in the potencies of tachykinin receptor antagonists in murine and human NK(3) receptors, and the lower potencies in the former should be taken into consideration when using murine disease models.

Genetic and pharmacological disruption of neurokinin 1 receptor function decreases anxiety-related behaviors and increases serotonergic function

Alterations in serotonin (5-hydroxytriptamine, 5-HT), norepinephrine, and gamma-aminobutyric acid have been linked to the pathophysiology of anxiety and depression, and medications that modulate these neurotransmitters are widely used to treat mood disorders. Recently, the neuropeptide substance P (SP) and its receptor, the neurokinin 1 receptor (NK1R), have been proposed as possible targets for new antidepressant and anxiolytic therapies. However, animal and human studies have so far failed to provide a clear consensus on the role of SP in the modulation of emotional states. Here we show that both genetic disruption and acute pharmacological blockade of the NK1R in mice result in a marked reduction of anxiety and stress-related responses. These behavioral changes are paralleled by an increase in the firing rate of 5-HT neurons in the dorsal raphe nucleus, a major source of serotonergic input to the forebrain. NK1R disruption also results in a selective desensitization of 5-HT1A inhibitory autoreceptors, which resembles the effect of sustained antidepressant treatment. Together these results indicate that the SP system powerfully modulates anxiety and suggest that this effect is at least in part mediated by changes in the 5-HT system.

Genetic mapping of the human C5a receptor. Identification of transmembrane amino acids critical for receptor function

Many hormones and sensory stimuli signal through a superfamily of seven transmembrane-spanning receptors to activate heterotrimeric G proteins. How the seven transmembrane segments of the receptors (a molecular architecture of bundled alpha-helices conserved from yeast to man) work as "on/off" switches remains unknown. Previously, we used random saturation mutagenesis coupled with a genetic selection in yeast to determine the relative importance of amino acids in four of the seven transmembrane segments of the human C5a receptor (Baranski, T. J., Herzmark, P., Lichtarge, O., Gerber, B. O., Trueheart, J., Meng, E. C., Iiri, T., Sheikh, S. P., and Bourne, H. R. (1999) J. Biol. Chem. 274, 15757-15765). In this study, we evaluate helices I, II, and IV, thereby furnishing a complete mutational map of the seven transmembrane helices of the human C5a receptor. Our analysis identified 19 amino acid positions resistant to non-conservative substitutions. When combined with the 25 essential residues previously identified in helices III and V-VII, they delineate two distinct components of the receptor switch: a ligand-binding surface at or near the extracellular surface of the helix bundle and a core cluster in the cytoplasmic half of the bundle. In addition, we found critical amino acids in the first and second helices that are predicted to face the lipid membrane. These residues form an extended surface that might mediate interactions with lipids and other membrane proteins or function as an oligomerization domain with other receptors.

Ligand recognition of serine-cysteine amino acid exchanges in transmembrane domain 5 of alpha2-adrenergic receptors by UK 14,304

Ligand binding of UK 14,304 reveals notable species (i.e., human-rodent) and receptor-subtype differences of alpha2-adrenergic receptors (alpha2-ARs). To study the molecular basis of the selectivity of UK 14,304, we compared a series of conservative serine-cysteine exchange mutants at ligand-accessible positions in transmembrane domain 5 of the human and mouse alpha2A-ARs. UK 14,304 bound with approximately 200-fold higher affinity to the human alpha2A-AR wild-type receptor compared with the human alpha2A-ARSer201 mutant, but only an approximately fivefold difference was seen with the corresponding mouse alpha2A-AR variant. These effects of cysteine-serine exchanges only involved the agonist low-affinity forms of the receptors, as the affinity of [3H]UK 14,304 for the agonist high-affinity receptor populations was not influenced. The apparent affinities of a set of eight structurally diverse alpha2-AR ligands (six agonists and two antagonists) were not influenced significantly by the cysteine-serine exchanges (except for oxymetazoline and yohimbine, with up to nine- and eightfold differences in affinity, respectively). We conclude that position 201 (a) plays a primary role in determining observed subtype/species selectivity of UK 14,304 in competitive antagonist radioligand binding assays and (b) does not determine the subtype selectivity of chlorpromazine.

Species selectivity of a small molecule antagonist for the CCR1 chemokine receptor

The species specificity of a small molecule antagonist for the human CCR1 chemokine receptor, 2-2-diphenyl-5-(4-chlorophenyl)piperidin-1-yl)valeronitrile (CCR1 antagonist 1), has been examined using cloned CCR1 receptors from various species. The compound was able to bind to rabbit, marmoset, and human CCR1, and was able to block the functional activation of these receptors. However, it failed to significantly displace radiolabeled macrophage inflammatory protein-1alpha (MIP-1alpha) binding to mouse CCR1 at concentrations up to 10 microM. These data suggested that the antagonist binding site is well-conserved in rabbit, marmoset and human CCR1, but not in mouse CCR1. The functional selectivity and mechanism of action for CCR1 antagonist 1 were further characterized. CCR1 antagonist 1 blocked the increase in intracellular Ca(2+) stimulated by CCR1 agonists, but had no effect on N-formyl-Met-Leu-Phe (FMLP), monocyte chemotactic protein-1 (MCP-1) and stromal-derived factor 1alpha (SDF1alpha)-induced Ca(2+) mobilization, demonstrating functional selectivity for CCR1. Since CCR1 antagonist 1 is a functional antagonist of marmoset and rabbit CCR1 receptors, it should be possible to test its efficacy in animal models of disease.

Uncovering molecular mechanisms involved in activation of G protein-coupled receptors

G protein-coupled, seven-transmembrane segment receptors (GPCRs or 7TM receptors), with more than 1000 different members, comprise the largest superfamily of proteins in the body. Since the cloning of the first receptors more than a decade ago, extensive experimental work has uncovered multiple aspects of their function and challenged many traditional paradigms. However, it is only recently that we are beginning to gain insight into some of the most fundamental questions in the molecular function of this class of receptors. How can, for example, so many chemically diverse hormones, neurotransmitters, and other signaling molecules activate receptors believed to share a similar overall tertiary structure? What is the nature of the physical changes linking agonist binding to receptor activation and subsequent transduction of the signal to the associated G protein on the cytoplasmic side of the membrane and to other putative signaling pathways? The goal of the present review is to specifically address these questions as well as to depict the current awareness about GPCR structure-function relationships in general.

Species-dependent pharmacological properties of the melanocortin-5 receptor

The genes encoding the melanocortin-3 receptor and melanocortin-5 receptor have been cloned from rhesus monkey. Heterologous expression in CHO cells indicated species dependent in vitro pharmacological properties for the human and rhesus melanocortin-5 receptors. Several peptides including NDP-alpha-MSH, alpha-MSH, MT-II and ACTH1-24 are more potent at the rhesus melanocortin-5 receptor than the human melanocortin-5 receptor by more than 10-fold. In contrast, we found no species difference in pharmacological properties between the human and rhesus melanocortin-3 receptors. Such a species-dependent pharmacological difference for melanocortin-5 receptor appears to be an exception compared to other G protein-coupled receptors from human and rhesus monkey.

Characterization of rodent liver and kidney AVP receptors: pharmacologic evidence for species differences

Radioligand binding studies with [3H]vasopressin (AVP) were used to determine the affinities of AVP receptor agonists and antagonists for mouse liver and kidney plasma membrane preparations. Both membrane preparations exhibited one class of high-affinity binding site. AVP ligand binding inhibition studies confirmed that mouse liver binding sites belong to the V1A subtype while kidney binding sites belong to the V2 receptor subtype. The affinity of each ligand for mouse V1A receptors was very similar to that for rat V1A receptors, showing differences in Ki values of less than 3-fold. In contrast, several peptide (d(CH2)5Tyr(Me)AVP) and nonpeptide (OPC-21268 and SR 49059) ligands had different affinities for mouse and rat kidney V2 receptors, with differences in Ki values ranging from 14- to 17-fold. These results indicate that mouse and rat kidney V2 receptors show significant pharmacologic differences.

The tachykinin NK1 receptor in the brain: pharmacology and putative functions

After its discovery in 1931, substance P (SP) remained the only mammalian member of the family of tachykinin peptides for several decades. Tachykinins thus refer to peptides sharing the common C-terminal amino acid sequence Phe-X-Gly-Leu-Met x NH2. In recent years the family of mammalian tachykinins has grown with the isolation of two novel peptides from bovine and porcine central nervous system (CNS), neurokinin A and neurokinin B. In parallel with the identification of multiple endogenous tachykinins several classes of tachykinin receptors were discovered. The receptors described so far are named tachykinin NK1 receptor, tachykinin NK2 receptor and tachykinin NK1 receptor, respectively. The present review focuses on the pharmacology and putative function of tachykinin NK1 receptors in brain. The natural ligand with the highest affinity for the tachykinin NK1 receptor is SP itself. The C-terminal sequence is essential for activity, the minimum length of a fragment with reasonable affinity for the tachykinin NK1 receptor is the C-terminal hexapeptide. A rapid advance of knowledge was caused by development of non-peptidic tachykinin NK1 receptor antagonists. This area is under rapid development and a variety of different chemical classes of compounds are involved. Species-dependent affinities of tachykinin NK1 receptor antagonists reveal two clusters of compounds, targeting the tachykinin NK1 receptor subtype found in guinea pig, human or ferret or the one in rat or mouse, respectively. The most recently developed compounds are highly selective, enter the brain and are orally bioavailable. Distinct behavioural effects in experimental animals suggest the involvement of tachykinin NK1 receptors in nociceptive transmission, basal ganglia function or anxiety and depression. Recent clinical trials in man showed that tachykinin NK1 receptor antagonists are effective in treating depression and chemotherapy-induced emesis. Therefore, it is well possible that tachykinin NK1 receptor antagonists will be clinically used for treatment of specific CNS disorders within a short period of time.

C5a receptor activation. Genetic identification of critical residues in four transmembrane helices

Hormones and sensory stimuli activate serpentine receptors, transmembrane switches that relay signals to heterotrimeric guanine nucleotide-binding proteins (G proteins). To understand the switch mechanism, we subjected 93 amino acids in transmembrane helices III, V, VI, and VII of the human chemoattractant C5a receptor to random saturation mutagenesis. A yeast selection identified 121 functioning mutant receptors, containing a total of 523 amino acid substitutions. Conserved hydrophobic residues are located on helix surfaces that face other helices in a modeled seven-helix bundle (Baldwin, J. M., Schertler, G. F., and Unger, V. M. (1997) J. Mol. Biol. 272, 144-164), whereas surfaces predicted to contact the surrounding lipid tolerate many substitutions. Our analysis identified 25 amino acid positions resistant to nonconservative substitutions. These appear to comprise two distinct components of the receptor switch, a surface at or near the extracellular membrane interface and a core cluster in the cytoplasmic half of the bundle. Twenty-one of the 121 mutant receptors exhibit constitutive activity. Amino acids substitutions in these activated receptors predominate in helices III and VI; other activating mutations truncate the receptor near the extracellular end of helix VI. These results identify key elements of a general mechanism for the serpentine receptor switch.

Identification of residues responsible for the selective binding of peptide antagonists and agonists in the V2 vasopressin receptor

To improve our understanding of the functional architecture of G protein-coupled receptors, we have taken advantage of differences among mammalian species in ligand binding to search for the rat versus human selectivity determinants of the V2 vasopressin receptor and of its peptide ligands. Our data indicate that residue 2 of species-selective peptide antagonists such as d(CH2)5-[D-Ile2,Ile4, Tyr-NH29]arginine vasopressin controls their rat versus human selectivity. For species-selective agonists such as desmopressin, residues 1 and 8 modulate the binding selectivity. Among residues different between rat and human V2 receptors, those localized in the upper part of the human V2 receptor have been substituted with their rat V2 homologs. Pharmacological analysis of mutant receptors revealed that residues 202 and 304 fully control the species selectivity of the discriminating antagonists in an independent and additive manner. A third residue (position 100) is necessary to observe an equivalent phenomenon for the discriminating agonists. The substitution of these three residues does not modify the affinity of the nonselective agonists and antagonists. In conclusion, extracellular loops and the top of the transmembrane domains of V2 vasopressin receptors may provide the molecular basis for peptide ligand-binding species selectivity. Very few residues in these regions may control the binding mode of both agonists and antagonists.

High affinity recognition of serotonin transporter antagonists defined by species-scanning mutagenesis. An aromatic residue in transmembrane domain I dictates species-selective recognition of citalopram and mazindol

Human and Drosophila melanogaster serotonin (5-HT) transporters (SERTs) exhibit similar 5-HT transport kinetics and can be distinguished pharmacologically by many, but not all, biogenic amine transporter antagonists. By using human and Drosophila SERT chimeras, major determinants of potencies of two transporter antagonists, mazindol and citalopram, were tracked to the amino-terminal domains encompassing transmembrane domains I and II. Species-scanning mutagenesis, whereby amino acid substitutions are made switching residues from one species to another, was employed on the eight amino acids that differ between human and Drosophila SERTs in this region, and antagonist potencies were reassessed in 5-HT uptake assays. A single mutation in transmembrane domain I of human SERT, Y95F, shifted both citalopram and mazindol to Drosophila SERT-like potencies. Strikingly, these potency changes were in opposite directions suggesting Tyr95 contributes both positive and negative determinants of antagonist potency. To gain insight into how the Y95F mutant might influence mazindol potency, we determined how structural variants of mazindol responded to the mutation. Our studies demonstrate the importance of the hydroxyl group on the heterocyclic nucleus of mazindol for maintaining species-selective recognition of mazindol and suggest that transmembrane domain I participates in the formation of antagonist-binding sites for amine transporters.

Chloroethylclonidine binds irreversibly to exposed cysteines in the fifth membrane-spanning domain of the human alpha2A-adrenergic receptor

The alpha2-adrenergic receptors (alpha2-ARs) mediate signals to intracellular second messengers via guanine nucleotide binding proteins. Three human genes encoding alpha2-AR subtypes (alpha2A, alpha2B, alpha2C) have been cloned. Several chemical compounds display subtype differences in their binding and/or functional activity. Site-directed mutagenesis and molecular modeling are new tools with which to investigate the subtype selectivity of ligands. In this study, we introduce a new approach to mapping of the binding site crevice of the human alpha2A-AR. Based on a three-dimensional receptor model, we systematically mutated residues 197-201 and 204 in the fifth transmembrane domain of the human alpha2A-AR to cysteine. Chloroethylclonidine, an alkylating derivative of the alpha2-adrenergic agonist clonidine, binds irreversibly to alpha2A-ARs by forming a covalent bond with the sulfhydryl side chain of a cysteine residue exposed in the binding cavity, leading to inactivation of the receptor. Irreversible binding of chloroethylclonidine was used as a criterion for identifying introduced cysteine residues as being exposed in the binding cavity. The results supported a receptor model in which the fifth transmembrane domain is alpha-helical, with residues Val197, Ser200, Cys201, and Ser204 exposed in the binding pocket. Residues Ile198, Ser199, Ile202, and Gly203 face the lipid bilayer of the plasma membrane. This approach emerges as a powerful tool for structural characterization of the alpha2-ARs.

The potency of the novel tachykinin receptor antagonist CGP49823 at rat and gerbil motoneurones in vitro

The novel tachykinin receptor antagonist CGP49823 ((2R,4S)-2-benzyl-1-(3,5-dimethylbenzoyl)-4-(quinolin-4-y lmethylamino)piperidine) has been compared with three other selective non-peptide tachykinin NK1 receptor antagonists. The drugs were tested as antagonists of the depolarization of spinal motoneurones induced by bath application of the selective tachykinin NK1 receptor agonist septide-(6-11) (300 nM) for 120 s at 15 min intervals. The antagonists were bath applied and the depolarizations were recorded from lumbar ventral roots of 7 to 12 day old rat and gerbil hemisected spinal cords in vitro. The gerbil preparation is considered to model the human species variant of the tachykinin NK1 receptor. With the exception of SR140333 ((S)-1-[2-[3-(3,4-dichlorophenyl)-1-[[3-(1-methylethoxy)phenyl]ace tyl]-3-piperidinyl]ethyl]-4-phenyl-1-azoniabicyclo[2.2.2]octane chloride), the antagonists were approximately thirty-fold more potent on gerbil preparations. The respective mean IC50 values from gerbil preparations produced by CP96345 ((2S-cis)-2-(diphenylmethyl)-N-[(2-methoxyphenyl)methyl]-1-azabicy clo[2.2.2]octan-3-amine), CGP49823, SR140333 and CP99994 ((2S-cis)-N-[(2-methoxyphenyl)methyl]-2-phenyl-3-piperidinamine) were, in microM +/- S.E. (n) 0.10 +/- 0.02 (6), 0.22 +/- 0.03 (6), 0.30 +/- 0.10 (5) and 0.38 +/- 0.02 (5) and the corresponding values from the rat preparations were 3.7 +/- 0.4 (5), 7.8 + 1.3 (5), 1.06 +/- 0.16 (6) and 10.5 +/- 2.2 (7). Dominance of tachykinin NK1 receptor activity in the measured responses was confirmed by low potency of the tachykinin NK2-selective antagonist SR48968 ((S)-N-methyl-N[4-(4-acetylamino-4-phenyl piperidino)-2-(3,4-dichlorophenyl)butyl] benzamide) which yielded an IC50 value of 12.0 +/- 2.8 (5) on gerbil preparations and produced less than 50% depression of septide-induced depolarization of rat motoneurones at the highest concentration (100 microM) tested.

The C9 methyl group of retinal interacts with glycine-121 in rhodopsin

The visual pigment rhodopsin is a prototypical G protein-coupled receptor. These receptors have seven transmembrane helices and are activated by specific receptor-ligand interactions. Rhodopsin is unusual in that its retinal prosthetic group serves as an antagonist in the dark in the 11-cis conformation but is rapidly converted to an agonist on photochemical cis to trans isomerization. Receptor-ligand interactions in rhodopsin were studied in the light and dark by regenerating site-directed opsin mutants with synthetic retinal analogues. A progressive decrease in light-dependent transducin activity was observed when a mutant opsin with a replacement of Gly121 was regenerated with 11-cis-retinal analogues bearing progressively larger R groups (methyl, ethyl, propyl) at the C9 position of the polyene chain. A progressive decrease in light activity was also observed as a function of increasing size of the residue at position 121 for both the 11-cis-9-ethyl- and the 11-cis-9-propylretinal pigments. In contrast, a striking increase of receptor activity in the dark-i.e., without chromophore isomerization-was observed when the molecular volume at either position 121 of opsin or C9 of retinal was increased. The ability of bulky replacements at either position to hinder ligand incorporation and to activate rhodopsin in the dark suggests a direct interaction between these two sites. A molecular model of the retinal-binding site of rhodopsin is proposed that illustrates the specific interaction between Gly121 and the C9 methyl group of 11-cis-retinal. Steric interactions in this region of rhodopsin are consistent with the proposal that movement of transmembrane helices 3 and 6 is concomitant with receptor activation.

The tachykinin NK1 receptor. Part I: ligands and mechanisms of cellular activation

The tachykinin NK1 receptor is widely expressed in the mammalian central and peripheral nervous system. Powerful pharmacological tools (agonists and antagonists) are now available to elucidate the physiological role of NK1 receptors at these levels, as well as to understand their role in diseases and establish the possible therapeutic usefulness of NK1 receptor antagonists for treatment of human diseases. The structure-activity studies that have led to the development of potent peptide and non-peptide ligands for the tachykinin NK1 receptor are here reviewed. Among the peptide agonists and antagonists, linear and cyclic sequences have been developed. The non peptide antagonists belong to different chemical classes, i.e. steroids, perhydroisoindolones, quinuclidines, piperidines and tryptophane derivatives. The first non peptide antagonists for NK1 receptors have been obtained by random screening of chemical compounds large collections. The resulting leads were optimized with 'classic' structure activity approaches, aiming at identifying 'common' motifs for interaction with the receptor by ligands of different chemical classes. The results derived from the recent application of molecular biology techniques were useful to drive the design of new ligands toward a precise structural definition of ligand-receptor bi-molecular interactions. Studies on mutant receptors have established that the sites of interaction of peptide agonists and non peptide antagonists with the tachykinin NK1 receptor are largely non overlapping. Moreover, data obtained from mutagenesis of the NK1 receptor further indicate that some amino acid residues in the NK1 receptor sequence are critical for determining the binding affinity of some but not all ligands. Therefore, different antagonists discovered from random screening may not possess common points of interaction or common structural and conformational characteristics for their interaction with the tachykinin NK1 receptor. The tachykinin NK1 receptor couples with G-proteins to determine its biological effects in target cells. Several G-proteins both sensitive (Go, Gi) and insensitive (Gq, G11) to pertussis toxin can mediate the action of NK1 receptors. Moreover, several second messanger signalling systems (elevation of intracellular calcium, stimulation of phosphoinositol turnover, arachidonic acid mobilization, cAMP accumulation) have to be activated following NK1 receptor signalling. Also a direct modulation of certain ion channels at membrane level has been proposed. The NK1 receptor undergoes prompt and significant tachyphylaxis upon exposure to the agonist: this has been shown to be linked with receptor internalization which also occurs physiologically when the NK1 receptor is stimulated by endogenous tachykinins.

Gly166 in the NK1 receptor regulates tachykinin selectivity and receptor conformation

We have studied the pharmacological properties of genetically engineered human NK1 tachykinin receptors in which residues at the extracellular surface of the fourth transmembranal domain were substituted with the corresponding amino acids from the NK2 receptor. We show that substitution of G166C:Y167F in the human NK1 receptor induces high affinity binding of a group of tachykinin ligands, known as 'septides' (i.e. neurokinin A, neurokinin B, [pGlu6,Pro9]-substance P6-11 and substance P-methylester). In contrast, binding of substance P and non-peptide antagonists is unaffected by these mutations. This effect parallels that found on the rat receptor and is therefore species specific. Second, we demonstrate that mutation of Gly166 to Cys alone is both necessary and sufficient to create this pan-reactive tachykinin receptor, whereas replacement of Tyr167 by Phe has no detectable effect on the pharmacological properties of the receptor. Furthermore, analysis of the effect of N-ethylmaleimide and dithiothreitol on binding of radiolabelled substance P documents differences in the mode in which this ligand interacts with wild-type and mutant receptors and supports the existence of a mutational induced change in the conformational status of the NK1 receptor.

Effect of single point mutations of the human tachykinin NK1 receptor on antagonist affinity

Molecular modelling and site-directed mutagenesis were used to identify eleven amino acid residues which may be involved in antagonist binding of the human tachykinin NK1 receptor. Recombinant receptors were expressed in mammalian cells using the Semliki Forest virus system. Wild type and mutant receptors showed similar expression levels in BHK and CHO cells, verified by metabolic labelling. Binding affinities were determined for a variety of tachykinin NK1 receptor antagonists in SFV-infected CHO cells. The binding affinity for GR203040, CP 99,994 and CP 96,345 was significantly reduced by mutant Q165A. The mutant F268A significantly reduced the affinity for GR203040 and CP 99,994 and the mutant H197A had reduced affinity for CP 96,345. All antagonists seemed to bind in a similar region of the receptor, but do not all rely on the same binding site interactions. Functional coupling to G-proteins was assayed by intracellular Ca2+ release in SFV-infected CHO cells. The wild type receptor and all mutants except A162L and F268A responded to substance P stimulation.

Mutagenesis and ligand modification studies on galanin binding to its GTP-binding-protein-coupled receptor GalR1

In this study, a large number of receptor mutants were generated and several N-terminally modified galanin analogues synthesized to refine the previously proposed binding site model for galanin to its GTP-binding-protein-coupled receptor GalR1. In addition to ligand-binding studies, the functionality of mutant receptors was evaluated by assessing their ability to mediate galaninergic inhibition of isoproterenol-stimulated adenylyl cyclase activity. The His264Ala and Phe282Ala receptor mutants, although deficient in binding in the concentration range of galanin used, remain functional albeit 20-fold less efficient than the wild-type receptor in mediating inhibition of stimulated cAMP production by galanin. The His267Ala mutant is, apart from being deficient in galanin binding, also severely impaired in functional coupling. While His264 and Phe282 seem to be important in forming the binding pocket for galanin, His267 might play a role in forming or stabilizing the active conformation of the GalR1 receptor rather than directly participating in the formation of the binding pocket for galanin. N-terminal carboxylic acid analogues of galanin have low affinity to wild-type GalR1, but substantially increased affinity to the Glu271Lys receptor mutant. This, together with the finding that an alanine substitution of Phe115 in TM III results in a tenfold decrease in affinity for galanin, suggests that the N-terminus of galanin interacts with Phe115. In contrast to the Phe282Ala mutation in TM VII, a conservative mutation of Phe282 to tyrosine did not alter the affinity for galanin. Thus, the interaction between Tyr9 of galanin and Phe282 is likely to be of an aromatic-aromatic nature.

Nonpeptide antagonists of neuropeptide receptors: tools for research and therapy

The recent development of selective and highly potent nonpeptide antagonists for peptide receptors has constituted a major breakthrough in the field of neuropeptide research. Following the discovery of the first nonpeptide antagonists for peptide receptors ten years ago, numerous other antagonists have been developed for most neuropeptide families. These new, metabolically stable compounds, orally active and capable of crossing the blood-brain barrier, offer clear advantages over the previously available peptide antagonists. Nonpeptide antagonists have provided valuable tools to investigate peptide receptors at the molecular, pharmacological and anatomical levels, and have considerably advanced our understanding of the pathophysiological roles of peptides in the CNS and periphery. Evidence from animal and clinical studies suggests that nonpeptide antagonists binding to peptide receptors could be useful for the treatment of disease states associated with high levels of neuropeptides. In this article Catalina Batancur, Mounia Azzi and William Rostène will address the recent developments in nonpeptide antagonists for neuropeptide receptors, with a particular focus on their CNS actions.

Further evidence for the involvement of tachykinin receptor subtypes in formalin and capsaicin models of pain in mice

The intradermal (i.d.) injection of NK1 receptor antagonists GR 82334 and FK 888 (1-50 pmol/paw), in association with formalin, produced graded inhibition of the early but not the late phase of the formalin test. The NK2, SR 48968 and NK3 SR 142801 receptor antagonists (1-50 pmol/paw) were effective in inhibiting both phases of the formalin model. Co-injection of NK1, (FK 888, GR 82334), NK2 (SR 48968) or NK3 (SR 142801) receptor antagonists with capsaicin dose-dependently attenuated capsaicin-induced licking. In addition, all antagonists were more efficacious when compared with response in the formalin test. The antinociception caused by i.d. injection of the NK3 receptor antagonist SR 142801 against both phases of the formalin test, but not that of NK1 and NK2 receptor antagonists, was significantly reversed by intraperitoneal (i.p.) injection of naloxone (5 mg/kg). Intracerebroventricular (i.c.v.) injection of NK1, NK2 or NK3 receptor antagonists (15-500 pmol/site), all produced significant and dose-dependent inhibition of both phases of the formalin and capsaicin tests. With the exception of the response of SR 48968, which was equipotent in both models of nociception, FK 888, GR 82334 and SR 142801 were about 2-25-fold less potent at the ID50 level against the capsaicin-induced pain. The antinociception caused by i.c.v. injection of NK1, NK2 or NK3 receptor antagonists was reversed by i.p. injection of naloxone (5 mg/kg). These results indicate that tachykinin receptor antagonists, acting through NK1, NK2 and NK3 receptors, produce powerful antinociception when injected i.d. or by i.c.v. route against both formalin- and capsaicin-induced licking, being more efficacious against the latter model of nociception. The action of NK3 receptor antagonist given i.d. was mediated through an opioid mechanism sensitive to naloxone. However, when injected i.c.v., the antinociception caused by NK1, NK2 or NK3 receptor antagonists was largely reversed by naloxone when assessed in the formalin test, suggesting a distinct mechanism of action.

Molecular characterization and functional expression of a substance P receptor from the sympathetic ganglion of Rana catesbeiana

Substance P is an important neuropeptide neurotransmitter in the central, autonomic and enteric nervous systems. In sympathetic ganglia, substance P is thought to play a role in modulating synaptic transmission. Release of substance P by neuronal stimulation or direct application of substance P to ganglionic neurons increases neuronal excitability. An amphibian substance P receptor complementary DNA has been cloned and characterized from bullfrog, Rana catesbeiana, sympathetic ganglion complementary DNA libraries. The deduced primary structure contains features indicative of a seven transmembrane domain G-protein-coupled receptor. The deduced protein sequence shows 69% identity to previously cloned mammalian substance P receptors. In situ hybridization analysis performed on bullfrog sympathetic ganglia using digoxigenin-labelled complementary RNA probe demonstrated that approximately 75% of the principal neurons displayed reaction product above background levels. Radioligand binding studies were performed on stably transfected cells with [(125)I]Tyr-1-substance P as the ligand. Substance P had an IC50 of 16 nM and the agonist potency profile was substance P>neurokinin A >> neurokinin B. The order of potency for three tachykinins to increase intracellular calcium when applied to a stably transfected clonal cell line was substance P>neurokinin A >> neurokinin B. This order of agonist potency also held for inhibition of the M-type potassium current in intact bullfrog sympathetic neurons. The non-peptide substance P antagonists CP-96345 and RP-67580 at concentrations that block mammalian substance P receptors had little or no effect on the responses to substance P at the bullfrog receptor. Overall, these results demonstrate that the cloned sequence has the features consistent with and characteristic of a substance P receptor. The results are discussed with reference to the established pharmacology of the bullfrog substance P receptor and known structure activity relationships of mammalian tachykinin receptors.

Improved discriminatory properties between human and murine tachykinin NK1 receptors of MEN 10930: a new potent and competitive antagonist

MEN 10930 (N alpha(N-[(1H)indol-3-yl-carbonyl]1-amino- cyclohexane-1-carbonyl)L-3-(2-naphthyl)alanine N-(benzyl) N methyl amide) interacts with high affinity with NK1 tachykinin receptor expressed in human IM9 (Ki = 1.0 +/- 0.17 nM) and U373MG (Ki = 2.8 +/- 0.5 nM) cells and guinea pig lung membranes (Ki = 5.9 +/- 0.8 nM). MEN 10930 shows no affinity for NK1 sites present in rat urinary bladder membranes up to 10 microM, resulting in more than 10,000-fold selectivity for the human NK1 receptor. In Scatchard plots performed in IM9 cells, MEN 10930 affects the substance P affinity, without changing the Bmax, suggesting a competitive interaction. It shows negligible affinity for calcium channels (Ki = 1.6 +/- 0.6 microM), NK2 receptor (Ki = 1.5 +/- 0.5 microM) and for NK3 receptor (Ki > 10 microM). Furthermore, MEN 10930 inhibits in a competitive manner the SP methyl ester-induced contractions in guinea pig ileum (pA2 = 8.7 +/- 0.08). In conclusion, MEN 10930 is a potent, selective, competitive antagonist of human, but not murine, NK1 receptor.

Identification of domains conferring ligand binding specificity to the prostanoid receptor. Studies on chimeric prostacyclin/prostaglandin D receptors

To identify domains conferring ligand binding specificity to prostanoid receptors, we constructed a series of chimeric receptors by successively replacing the regions from the carboxyl-terminal tail of mouse prostacyclin (prostaglandin I (PGI)) receptor (mIP) with the corresponding regions of the mouse PGD receptor (mDP). The mIP receptor expressed in COS 7 cells bound [3H]iloprost, a PGI2 analog, and [3H]PGE1 with Kd values of 13 and 27 nM, respectively. This receptor did not bind [3H]PGD2, [3H]PGE2, and [3H]PGF2alpha. The mDP receptor bound only [3H]PGD2 with a Kd value of 43 nM. The chimeric IPN-VII/DPC receptor with replacement of the carboxyl tail of the mIP receptor with that of the mDP receptor showed 12-16-fold higher affinities for [3H]iloprost and [3H]PGE1 than the mIP receptor. The region extending from the sixth transmembrane domain to the carboxyl terminus of the mIP receptor was next replaced with the corresponding region of the mDP receptor. This chimeric IPN-V/DPVI-C receptor acquired the ability to bind [3H]PGD2 and [3H]PGE2 without decreasing the affinities of the mIP receptor to [3H]iloprost and [3H]PGE1. These binding characteristics did not change when the fourth and fifth transmembrane domains of the mIP receptor were further replaced with the corresponding regions of the mDP receptor. However, when the first extracellular to second intracellular loop of the mIP receptor containing the third transmembrane domain was further replaced with those of the mDP receptor, the affinities for [3H]PGE1, [3H]PGE2, and [3H]iloprost were markedly decreased, whereas that for [3H]PGD2 was increased by about 2-fold. [3H]PGF2alpha showed no affinity for the mIP, mDP, and all the chimeric receptors. These results suggest that the sixth to seventh transmembrane domain of the mIP receptor confers the specificity of this receptor to bind selectively to PGE1 and not to PGE2 and that the third transmembrane domain of the mDP receptor confers the selective binding of PGD2 to this receptor.

Postnatal development of delta-opioid receptor subtypes in mice

1. The density and affinity of binding sites for the delta-selective opioid ligands [3H]-[D-Ala2, Asp4]deltorphin (DELT-I), [3H]-[D-Ala2Glu4]-deltorphin (DELT-II), [3H]-[D-Pen2,D-Pen5]enkephalin (DPDPE), and [3H]-naltrindole (NTI) were determined in whole brain from 10, 15, 25 and 60 day-old C57BL mice. 2. At all ages, the analyses of the homologous displacement curves, gave best fits to single rather than to multiple site models. The binding capacity (Bmax) labelled by [3H]-NTI was about one half that labelled by [3H]-DELT-I, [3H]-DELT-II and [3H]-DPDPE. In 25 and 60 day-old mouse brain the DPDPE Bmax was 25% less than the deltorphin-II Bmax. 3. In saturation experiments, specific binding of [3H]-DELT-I on adult mouse brain homogenates was best fitted by a two-site model (34%, high affinity site, Kd = 1.08 nM and 66% low affinity sites, Kd = 39.9 nM). 4. DPDPE produced a biphasic inhibition of specific [3H]-DELTI-I binding, from 15 days of age onwards. The relative percentage of high and low affinity sites was 72% and 28% in 15 day-, 65% and 35% in 25 day- and 30% and 70% in 60 day-old mice. 5. In adult mouse brain labelled with [3H]-DELT-I, DELT-II recognized 71% of high-affinity and 29% of low-affinity sites DELT-I and DPDPE produced monophasic inhibition of specific [3H]-DELT-II binding to brain homogenates of adult mice. 6. These data suggest that a sub-population of delta-sites (probably the delta 2-subtype), recognized by DELT-I, with high affinity for DELT-II and low affinity for DPDPE develops from 25 days onward. 7. In electrically stimulated mouse vas deferens (MVD) the rank order of potency of the three delta-agonists was: DELT-I > DELT-II > DPDPE in 10 day-old mice: and DELT-I- DELT-II > DPDPE, from 25 days onward. During this time, the potency of DELT-II increased about 15 fold whereas the potency of DELT-I and DPDPE increased only 5 times. The higher efficacy of DELT-II could depend on receptor maturation towards the delta 2-subtype.

The sixth transmembrane domains of the human B1 and B2 bradykinin receptors are structurally compatible and involved in discriminating between subtype-selective agonists

In order to investigate the molecular basis for the ability of the human B1 and B2 bradykinin (BK) receptor subtypes to discriminate between subtype-selective ligands, we constructed chimeric proteins in which the sixth transmembrane domains (TM-VI) of these receptors were exchanged. The pharmacological profiles of the constructs were analyzed by radioligand binding in particulate preparations of transiently transfected HEK293 cells using the agonist [3H]des-Arg10-kallidin and the antagonist [3H]NPC17731. The ability of these constructs to transmit an intracellular signal was measured in transiently transfected A10 cells, a vascular smooth muscle cell line, by single cell Ca2+ imaging. Substitution of B1 TM-VI into the B2 receptor (B2(B1VI)) dramatically reduced the affinity of the B2-selective agonist BK, whereas the affinity of the B2-selective antagonist NPC17731 was unaltered. High affinity BK binding was fully regained when two residues, Tyr259 and Ala263, near the extracellular surface of TM-VI in B2(B1VI), were replaced with the corresponding residues in the wild-type B2 receptor, which are Phe259 and Thr263. The construct B1(B2VI), produced by substitution of B2 TM-VI into the B1 receptor, did not support high affinity binding of the B1-selective agonist des-Arg10-kallidin. In contrast to BK and des-Arg10-kallidin, the binding of the less subtype-selective agonist kallidin showed little sensitivity to TM-VI exchange. These results show that TM-VI in the human B1 and B2 BK receptor subtypes, although only 36% identical, are structurally compatible. Furthermore, this domain contributes significantly to the ability of these receptors to discriminate between the subtype-selective agonists BK and des-Arg10-kallidin.

Engineering of metal-ion sites as distance constraints in structural and functional analysis of 7TM receptors

G-protein-coupled receptors with their seven transmembrane (7TM) segments constitute the largest superfamily of proteins known. Unfortunately, still only relatively low resolution structures derived from electron cryo-microscopy analysis of 2D crystals are available for these proteins. We have used artificially designed Zn(II) metal-ion binding sites to probe 7TM receptors structurally and functionally and to define some basic distance constraints for molecular modeling. In this way, the relative helical rotation and vertical translocation of transmembrane helices TM-II, TM-III, TM-V, and TM-VI of the tachykinin NK-1 receptor have been restricted. Collectively, these zinc sites constitute a basic network of distance constraints that limit the degrees of freedom of the interhelical contact faces in molecular models of 7TM receptors. The construction of artificially designed metal-ion sites is discussed also in the context of probes for conformational changes occurring during receptor activation.

Effect of substance P and receptor antagonists on secretion of lingual lipase and amylase from rat von Ebner's gland

Substance P (SP, 1 microM) when incubated with minced von Ebner's glands for 15, 30, and 60 min, stimulated secretion of lingual lipase (12.14% +/- 0.90) and amylase (8.30% +/- 0.42). Only 10 microM of the SP receptor antagonist CP-96,345 significantly inhibited SP-evoked secretion. D-Pro2-D-Phe7-D-Trp9-SP (Ia), D-Pro2-D-Trp7,9-SP (Ib), D-Arg1-D-Trp7,9-D-Leu11-SP (Ic), or 1 microM CP-96,345 were not effective, suggesting that the SP receptor of von Ebner's gland might be an isoform. Propranolol and timolol, beta 1/beta 2-adrenergic receptor antagonists were not effective and the cholinergic receptor antagonist, atropine, was effective in only slightly reducing amylase secretion but not lingual lipase. Differential secretion of the two enzymes was observed for basal and stimulated secretion. Thus, exocytosis may not be the only pathway involved in SP-evoked protein secretion.

The effects of amino acid replacements of glycine 121 on transmembrane helix 3 of rhodopsin

Rhodopsin is a member of a family of G protein-coupled receptors with seven transmembrane (TM) helices. In rhodopsin, Gly121 is a highly conserved amino acid residue near the middle of TM helix 3. TM helix 3 is known to be involved in chromophore-protein interactions and contains the chromophore Schiff base counterion at position 113. We prepared a set of seven single amino acid replacement mutants of rhodopsin at position 121 (G121A, Ser, Thr, Val, Ile, Leu, and Trp) and control mutants with replacements of Gly114 or Ala117. The mutant opsins were expressed in COS cells and reconstituted with either 11-cis-retinal, the ground-state chromophore of rhodopsin, or all-trans-retinal, the isomer formed upon receptor photoactivation. The replacement of Gly121 resulted in a relative reversal in the selectivity of the opsin apoprotein for reconstitution with 11-cis-retinal over all-trans-retinal in COS cell membranes. The mutant pigments also were found to be thermally unstable to varying degrees and reactive to hydroxylamine in the dark. In addition, the size of the residue substituted at position 121 correlated directly to the degree of blue-shift in the lambdamax value of the pigment. These results suggest that Gly121 is an important and specific component of the 11-cis-retinal binding pocket in rhodopsin.

L-733,060, a novel tachykinin NK1 receptor antagonist; effects in [Ca2+]i mobilisation, cardiovascular and dural extravasation assays

This study investigated the properties of a novel piperidine ether-based tachykinin NK1 receptor antagonist L-733,060, ((2S,3S)-3-((3,5-bis(trifluoromethyl)phenyl)methyloxy)-2-phenyl piperidine and its 2R,3R-enantiomer L-733,061 on [Ca2+]i mobilisation in Chinese hamster ovary cells transfected with human tachykinin NK1 receptors, compared to their effects in rodent cardiovascular and neurogenic plasma extravasation assays. Using FURA-2-imaging techniques, L-733,060 inhibited substance P-induced [Ca2+]i mobilisation with an estimated affinity of 0.8 nM whereas L-733,061 (30-300 nM) did not. No significant effects of L-733,060 were observed on mean arterial blood pressure or heart rate in conscious or anaesthetised rats at doses of < 3000 micrograms kg-1 i.v. L-733,060 also stereoselectively inhibited neurogenic plasma extravasation in rat dura produced by electrical stimulation of trigeminal nerves with an ID50 of 212 +/- 19 micrograms kg-1 i.v. Thus, L-733,060 is a novel antagonist of human tachykinin NK1 receptors which stereoselectively inhibits neurogenic plasma extravasation at doses that do not cause adverse cardiovascular effects.

The use of photolabelled peptides to localize the substance-P-binding site in the human neurokinin-1 tachykinin receptor

The amino acid p-benzoyl-L-phenylalanine, (p-Bz)Phe, has been incorporated into substance P (SP), Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2, to localize the agonist-binding domains of the human neurokinin-1 (NK-1) receptor overexpressed in a transfected mammalian cell line. The NK-1-specific agonist [Pro9]SP was modified at position 8 by (p-Bz)Phe and acylated at the N-terminus by a biotinyl sulfone reporter via a 5-aminopentanoyl spacer. After photolysis, the biotinyl sulfone moiety allowed easy and efficient removal of biotinylated fragments from the complex incubation mixture with streptavidin-coated beads. Direct elution from the beads with the matrix used for matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOFMS), which was facilitated by saturation of streptavidin sites with biotin, and subsequent MALDI-TOF mass spectrometry analysis allowed identification of the NK-1 fragments obtained after photolysis and proteolytic digestion. Trypsin digestion and combined trypsin/Staphylococcus aureus V8 protease enzymatic cleavage established that the site of covalent attachment of the photolabelled SP resides in the second extracellular loop Thr173-Arg177. Cyanogen bromide cleavage shows that the probe is covalently attached to the methyl group of a methionine residue from human NK-1. These experiments identified Met174 as the modified residue.

Pharmacological evidence that NK-2 tachykinin receptors mediate hypotension in the guinea pig but not in the rat

Vascular NK-1 and NK-2 tachykinin receptors in the rat and the guinea pig were characterized pharmacologically by using available agonists and antagonists exhibiting varying degrees of selectivity for these receptors. Because the anesthetized guinea pig has unusually low blood pressure, these animals were pithed and vagotomized and infused, throughout the experimental procedure, with norepinephrine (6 micrograms/kg/min). This treatment raised their blood pressure to a level appropriate for the determination of dose-hypotensive response curves. The NK-1 receptor agonists substance P (SP) and septide (0.004 to 1 microgram/kg iv) decreased carotid artery blood pressure in a dose-dependent manner in both species, but they were more potent (13- and 33-fold, respectively) in guinea pigs than in rats. The NK-2 receptor agonist [beta Ala8]-NKA(4-10) (0.06 to 1 microgram/kg) also dose-dependently lowered blood pressure in the pithed guinea pig with noradrenaline-supported blood pressure, although it failed to do so in the same rat preparation. RP 67580, a selective NK-1 antagonist, antagonized the SP- or septide-induced hypotensive response in rats, but not in guinea pigs. Conversely, RPR 100893, a novel NK-1 receptor antagonist chemically related to RP 67580, dose-dependently inhibited hypotension induced by SP, and even more, that induced by septide only in guinea pigs. In the latter species, neither RP 67580 nor RPR 100893 affected decreases in blood pressure induced by [beta Ala8]-NKA(4-10). These decreases were, however, inhibited by the NK-2 receptor antagonist SR 48968. The selectivity of this compound for the latter receptor was confirmed by its failure to affect SP- or septide-induced hypotension in either guinea pigs or rats. These results confirm that the hypotensive responses to SP and septide are mediated by NK-1 receptors in the two species studied. However, functional NK-2 receptors appear to be present in the vascular bed of the guinea pig but not that of the rat, since in the former species the hypotensive responses induced [beta Ala8]-NKA(4-10) were inhibited by SR 48968 but not by the NK-1 receptor antagonists studied. This conclusion is, to our knowledge, drawn here for the first time from clear-cut pharmacological results.

Tachykinins: receptor to effector

Tachykinins belong to an evolutionarily conserved family of peptide neurotransmitters. The mammalian tachykinins include substance P, neurokinin A and neurokinin B, which exert their effects by binding to specific receptors. These tachykinin receptors are divided into three types, designated NK1, NK2 and NK3, respectively. Tachykinin receptors have been cloned and contain seven segments spanning the cell membrane, indicating their inclusion in the G-protein-linked receptor family. The continued development of selective agonists and antagonists for each receptor has helped elucidate roles for these mediators, ranging from effects in the central nervous system to the perpetuation of the inflammatory response in the periphery. Various selective ligands have shown both inter- and intraspecies differences in binding potencies, indicating distinct binding sites in the tachykinin receptor. The interaction of tachykinin with its receptor activates Gq, which in turn activates phospholipase C to break down phosphatidyl inositol bisphosphate into inositol trisphosphate (IP3) and diacylglycerol (DAG). IP3 acts on specific receptors in the sarcoplasmic reticulum to release intracellular stores of Ca2+, while DAG acts via protein kinase C to open L-type calcium channels in the plasma membrane. The rise in intracellular [Ca2+] induces the tissue response. With an array of actions as diverse as that seen with tachykinins, there is scope for numerous therapeutic possibilities. With the development of potent, selective non-peptide antagonists, there could be potential benefits in the treatment of a variety of clinical conditions, including chronic pain, Parkinson's disease, Alzheimer's disease, depression, rheumatoid arthritis, irritable bowel syndrome and asthma.

Effect of the tachykinin receptor antagonists, SR 140333, FK 888, and SR 142801, on capsaicin-induced mouse ear oedema

We examined the effect of SR 140333, a nonpeptide NK1 receptor antagonist, FK 888, a peptide NK1 antagonist, and SR 142801, a non-peptide NK3 antagonist, on ear oedema induced by topical application of capsaicin (250 micrograms/ear) in mice. SR 140333 (ED50:39 micrograms/kg, i.v.) dose-dependently inhibited the oedema response to capsaicin, whereas FK 888 (1.0 mg/kg, i.v.) and SR 142801 (3.0 mg/kg, i.v.) had no effect. Furthermore, SR 140333 significantly (p < 0.001) suppressed ear oedema in response to intradermal injection of substance P (SP) (100 pmol/site) by i.v. administration (0.1 mg/kg,) and co-injection (50 pmol/site). In contrast, FK 888 (1.0 mg/kg, i.v. and 500 pmol/site) was ineffective in the response to SP. The present results suggest that the difference in effects of the two NK1 receptor antagonists on the oedema response to capsaicin is due to species differences in affinities for the NK1 receptor in the mouse skin. Moreover, it seems unlikely that the NK3 receptor is involved primarily in capsaicin-induced mouse ear oedema.

His381 of the rat CCKB receptor is essential for CCKB versus CCKA receptor antagonist selectivity

A great interest is devoted to antagonists of the cholecystokinin type B (CCKB) receptor such as L-365,260, which reduces panic attacks in humans and to antagonists of the cholecystokinin type A (CCKA) receptor, such as L-364,718 which might be efficient in mental diseases. The A/B specificity of these antagonists was proposed to be mainly dependent on the amino acid sequence of the seventh transmembrane domain (Mantamadiotis and Baldwin (1994) Biochem. Biophys. Res. Commun. 201,1382). In our study, one of these residues, His381 was replaced in the rat CCKB receptor by leucine (the corresponding residue in the CCKA receptor), phenylalanine or arginine using site-directed mutagenesis. Changing histidine for leucine or phenylalanine did not modify significantly the affinity of the CCKB receptor antagonists, L-365,260 and PD-134,308 although both compounds belong to different chemical classes, but strongly improved the affinity of the CCKA receptor antagonists tested. Interestingly, the A selectivity of these CCKA receptor antagonists was recovered by substituting His381 by arginine. Moreover, these results are discussed on the basis of a three dimentional model of the CCKB receptor. The mutated receptors possessed unchanged binding properties for agonists, suggesting that determinants confering specificity for agonists and antagonists are different.

In vitro and in vivo pharmacology of S 16474, a novel dual tachykinin NK1 and NK2 receptor antagonist

Since tachykinins released from lung sensory nerve endings are thought to play a role in inflammatory diseases of airways via NK1 and NK2 receptors, dual tachykinin NK1 and NK2 receptor antagonists may have a great therapeutic potential. In vitro, the cyclopeptide S 16474 (cyclo-[Abo-Asp(D-Trp(Suc0Na)-Phe-N-(Me)Bzl)]) bound to both human tachykinin NK1 and NK2 receptors expressed in two lines of transfected Chinese hamster ovary cells (IC50 values 85 nM and 129 nM, respectively), while showing a poor affinity for the rat tachykinin NK1 receptor. S 16474 inhibited the contractions induced by substance P in isolated rabbit vena cava (pA2 7.0) and by neurokinin A in rabbit pulmonary artery (pA2 5.6). In vivo in anaesthetized guinea-pigs, S 16474 was found to dose dependently inhibit the bronchoconstrictions induced by intravenously administered substance P, neurokinin A and capsaicin. Plasma extravasation evoked in bronchi by endogenously released tachykinins under vagus nerve stimulation was abolished by S 16474 (10 mu mol/kg i.v.). These results demonstrate clearly that S 16474 is a tachykinin receptor antagonist exhibiting, in vitro and in vivo, a dual inhibitory effect on NK1 and NK2 receptors.

Temperature and agonist dependency of tachykinin NK1 receptor antagonist potencies in rat isolated superior cervical ganglion

Using rat isolated superior cervical ganglion we have further characterised tachykinin NK1 receptors and investigated the possible existence of tachykinin NK1 receptor subtypes. At 37 degrees C, tachykinin NK1 receptor antagonists GR82334 ([D-Pro9[spiro-gamma- lactam]Leu10,Trp11]physalaemin-1(1-11)), CP-99,994 ((+)-(2S,3S)-3-(2-methoxybenzylamino)-2-phenylpiperidine) and (+/-)-RP67580 (7,7-diphenyl-2[1-imino-2(2-methoxy- phenyl)-ethyl]perhydroisoindol-4-one (3aR,7aR)) antagonised more potently depolarisation responses evoked by GR73632 (delta Ava]L-Pro9,N-MeLeu10]SP-(7-11)), septide ([pGlu6,Pro9]SP-(6-11)) and neurokinin A than those evoked by substance P, substance P O-methyl ester and [Sar9,Met(O2)11]substance P. GR73632 and substance P O-methyl ester evoked depolarisation responses of similar magnitude, unaffected by addition of tetrodotoxin, but which cross-desensitised. At 22 degrees C, the ability of GR82334 and (+/-)-RP67580 to inhibit substance P O-methyl ester-evoked but not GR73632-evoked responses was enhanced greatly. These results suggest a single population of tachykinin NK1 receptors in this preparation. The agonist and temperature dependency of tachykinin NK1 receptor antagonist potency in rat isolated superior cervical ganglion may reflect different conformational changes in the tachykinin NK1 receptor induced by partial or full sequence substance P analogues.

Recent advances in molecular recognition and signal transduction of active peptides: receptors for opioid peptides

1. Opioid peptides are a family of structurally related neuromodulators which play a major role in the control of nociceptive pathways. These peptides act through membrane receptors of the nervous system, defined as mu, delta and kappa and endowed with overlapping but distinct pharmacological, anatomical and functional properties. 2. Recent cloning of an opioid receptor gene family has opened the way to the use of recombinant DNA technology at the receptor level. 3. This review focuses on the molecular cloning and functional characterization of opioid receptors and provides first insights into molecular aspects of opioid peptide recognition and signal transduction mechanisms, using the cloned receptors as investigation tools.

Leukotriene receptors

The current challenge in research on leukotriene receptors is to clone these molecules. Traditional protein purification approaches have not been successful in providing sequence information. Solubilization of cys-LT1 has been achieved but results in the dissociation of G-proteins and the loss of high affinity binding (Mong et al., 1986b; Mong and Sarau, 1990), while cys-LT2 activity cannot be monitored by other than functional assays and there have not been any purification attempts. Partial purification of B-LT has been reported but has not been continued to homogeneity (Sherman et al., 1992; Votta et al., 1990; Miki et al., 1990). Nor have attempts to clone these receptors through either homology screening or expression cloning been successful. The cloning of the prostanoid receptors, described in detail elsewhere in this volume, has shown that these receptors belong to a distinct family within the G-protein-coupled receptor superfamily. It is probable, therefore, that the leukotriene receptors will also belong to a separate group within this superfamily since phylogenic comparisons have shown that receptors displaying high affinity for structurally related ligands exist as discrete families. Recently, a human cDNA encoding an orphan FMLP-related receptor cloned from HL60 cells of myeloid lineage was identified as the receptor for another eicosanoid, lipoxin A (Fiore et al., 1994). FMLP has a similar profile of biological actions to LTB4. Moreover, LTD4 showed a high degree of cross-reactivity with this receptor with an affinity only 20-fold less that of lipoxin A, although LTB4 was inactive. It remains to be determined whether the leukotriene receptors will fall into this class of receptors. The cloning of the leukotriene receptors will allow identification of the different receptor types and subtypes and potentially splice variants. Evaluation of currently developed antagonists at these receptor types could also open the way for novel therapies for inflammatory conditions.

Cloning, sequencing, and expression of the rhesus monkey beta 2 adrenergic receptor

The beta 2 adrenergic receptor (beta 2AR) plays a key role in the signal transduction mechanism for epinephrine and norepinephrine. The gene for beta 2 adrenergic receptors has been cloned for several species, but has remained undetermined for rhesus monkey. In this study, we report the isolation of the gene encoding the rhesus beta 2AR from both cDNA and genomic DNA sources. Sequence analysis of the gene reveals an intronless open reading frame that encodes a 415-amino-acid protein. The rhesus receptor is highly homologous to that from other species, especially to the human receptor (97% sequence identity). Functional characterization by ligand binding and agonist-mediated cAMP accumulation indicates that the rhesus beta 2 receptor shares a very similar pharmacological profile with the human beta 2 receptor. Therefore, the rhesus monkey represents a valid animal model for developing therapeutic agents targeted at the corresponding human beta 2 receptor.

The mammalian tachykinin receptors

The tachykinins (TKs) are a family of small peptides which share the common C-terminal sequence Phe-X-Gly-Leu-MetNH2. Three peptides of this family, substance P, neurokinin A and neurokinin B, have an established role as neurotransmitters in mammals. 2. Three receptors for TKs have been cloned: they are G-protein coupled receptors with seven putative transmembrane spanning segments and have been termed NK1 (substance P-preferring), NK2 (neurokinin A-preferring) and NK3 (neurokinin B-preferring). 3. Synthetic agonists are available to selectively stimulate only one receptor, while natural TKs can act as full agonist at each one of the three receptors, albeit at different concentrations. 4. A number of potent and selective antagonists, both peptide and nonpeptide in nature, have recently been developed. 5. The introduction of these ligands has revealed an unforeseen pharmacological heterogeneity of NK1, NK2 and NK3 receptors which appears largely, if not exclusively, linked to the existence of species homologues of the three receptors.

Central versus peripheral site of action of the tachykinin NK1-antagonist RP 67580 in inhibiting chemonociception

1. Many studies indicate an involvement of substance P in the transmission of nociceptive stimuli, without, however, presenting any conclusive evidence as to its exact site and mode of action. The present experiments tested the involvement of substance P in the mediation of chemical nociception using the non-peptidic specific tachykinin NK1-receptor antagonist, RP 67580 (2-[1-imino-2-(2-methoxyphenyl-ethyl]-7,7diphenyl-4-perhydroiso indolone (3aR, 7aR)). 2. Mean arterial pressure (MAP) and intragastric pressure (IGP) were measured in anaesthetized rats. The reflex changes of these parameters in response to i.p. or s.c. injections of hydrochloric acid or capsaicin were taken to indicate nociception. 3. Intravenous administration of RP 67580 up to 5 mg kg-1 had little influence on the reflex changes in MAP or IGP in response to hydrochloric acid or capsaicin. In contrast, the sensitization of rats to i.p. capsaicin by preinjection of prostaglandin E2 was significantly reduced by 1 mg kg-1 RP 67580. 4. Intrathecal injection of 5 micrograms RP 67580 inhibited the reflex changes of MAP and IGP in response to i.p. or s.c. capsaicin whereas the inactive enantiomer RP 68651 was ineffective. 5. The results indicate that spinal NK1-receptors are involved in the acute transmission of chemically induced pain, while such receptors in the periphery take part in the sensitization by prostaglandin E2. The rather minor ability of i.v. RP 67580 to inhibit the acute nociceptive reflex is attributed to an insufficient penetration of the blood-brain-barrier.

Tachykinin receptors in gastrointestinal motility

For a long time research on the action of TKs on gastrointestinal tissue has been demonstrating the importance of the TKs as non-cholinergic stimulators of motility in most parts of the mammalian gastrointestinal tract. The past years witnessed the development of TK agonists and antagonists selective for the various receptor types, which prompted a wealth of new insight into the pharmacology and molecular biology of the TK receptors. This knowledge now allows a more specific elucidation of the role of TKs and their receptors in the various aspects of gastrointestinal motility, not only in normal tissue but also under pathological conditions.

Human N-methyl-D-aspartate receptor modulatory subunit hNR3: cloning and sequencing of the cDNA and primary structure of the protein

Several cDNA clones encoding the human N-methyl-D-aspartate receptor modulatory subunit hNR3, were isolated from a human fetal brain library. The hNR3 cDNA demonstrated a 91.3-91.5% nucleotide (nt) identity with the rat NR2B and mouse epsilon 2 cDNAs. The nt sequence of hNR3 would encode a 1484 amino acid (aa) protein that has a 98.4-98.5% identity with the mouse epsilon 2 and rat NR2B subunits.